...is between the period of 1 and 6 days after it's birth - because that is the way it knows that it's mother loves it. But what is love?
Love is the continued functioning of a particular gene in nervous cells in the hippocampus of the baby rat. A loved rat will go on through life, leading a self assured existence. And when it has children of it's own, you can be damn sure it'll lick those puppies and pass that love on.
But the neurotic mother, she doesn't give a lick to her children. She neglects them, and does not love them they way a good rat mother should. And what is the absence of love to a rat? It is the methylation of that same genome in the hippocampus of those rats; which, by the way, is a part of the brain common to all mammals.
This rat will go through life like a neurotic wreck, and when her children are born, she will not love them the way she is supposed to, by licking them.